Markarian (Mkn) 421 and Mkn 501 are two relatively nearby (z=0.034) galaxies about 300-500 light years (0.5 Gly) away. Mkn 501 is a bright optical object (blazar) in the active galactic nuclei (AGN) of a giant elliptical (young) galaxy, containing probably black hole binary, which exhibit the high energy cutoffs predicted to be the result of intergalactic annihilation interactions, primarily with infrared photons. In accordance with this, a MAGIC telescope collaboration published an observation of 30 sec delay of two minutes lasting 10 TeV gamma ray gamma ray bursts (GRB) from summer 2005 with respect to low energy signal, i.e. high energy tail, which On the origin of highest energy gamma-rays from Mkn 501 can be explained by intergalactic cascade of gamma rays. Without presence of external gravity/magnetic field it could be explained by several models of quantum gravity as a result of dispersion of gamma rays by quantum foam structure of vacuum in 1o+7 eV scale.
With compare to this, the GZK limit considers dispersion by CMB photons at 1o+19 eV scale only. It was proven by recent 16 seconds lasting huge burst GRB 080916C from September 2008, as observed by Fermi / GLAST observatory which is working at 300 GeV scale, during which six-second delay separating the highest-energy emissions (red) from the lowest (white dots) was observed. The distance was estimated by wavelength of afterglow drop-off caused by intervening gas clouds (afterglow photometric redshift) to 12.2 Gly (z=4.35), which places GRB 080916C among the top 5% most distant GRBs, and makes it the most energetic GRB known to date.
While these results were interpreted by string theory proponents as a negative result of Lorentz symmetry violation immediately, we should realize, dispersion is exponential function of wavelength, the energy scale of Fermi/GLAST observatory is 30x lower, then MAGIC observatory and the GRB event was 8x shorter - so that the five-times shorter delay is still comfortably acceptable here. But if we still consider, GRB 080916C was 25x more distanced, then Mkn 501 blast it's apparent, the mechanism of high energy tail is still the same, i.e. of intergalactic origin, not a result of gamma ray photons dispersion in extragalactic space without presence of magnetic and gravitational fields.
By AWT this behavior is a consequence of nonlinear behavior of Aether foam (compare some ad-hoced QG models) and the same phenomena we can met at the water surface, where waves of different frequency are moving by different speed (compare the celerity curve), but burst propagates here at large distance like self-reinforcing solitary vortex ring or wave packet, i.e. the kink soliton. For soliton is typical quantization and fragmentation of time arrows (analogous to splitting of event horizon in Kerr metric of rotating black holes). When a wave is too big for the depth of water, it splits into two, one big and one small, which continue independently, i.e. no dispersion occurs here, until whole pulse propagates like single wave packet described by Sine-Gordon/Klein-Gordon wave equation (compare the "rubber band" model of soliton and/or special relativity). All particles including neural waves are sort of hyperdimensional solitons, while these of non-zero rest mass are so-called "breathers".
Soliton character of gamma ray bursts is the reason, why we can observe them at all without apparent dispersion even at cosmological distances, thus violating the Greisen-Zatsepin-Kuzmin limit and they can explain, how X-ray or gamma-ray luminosities are related to peak energy output or the luminosity at lower frequencies alternativelly to dual axion models (axions or gravitophotons are vortex analogy of burst solitons by AWT). In accordance with this apparently weaker burst of AGN Mkn 501 exhibits more apparent dispersion assigned to ALP mixing (i.e. axion coupling), then the sharper GRB 080916C, which exhibits Gaussian distribution, typical for solitons. By this way, AWT can explain even violation of Lorentz symmetry violation, i.e. 2nd order violation of relativity, which manifest itself only during spreading of cosmological energy densities at cosmological distance, thus doubly fooling some special relativity and/or string theory fundamentalists.